Large magnitude dynamic and static longitudinal, lateral, vertical, and torsional loads are developed by the main rotor assembly of a helicopter. Helicopter design methodology utilizes a support structure to integrate elements of the main rotor assembly such as the static rotor mast and the engine transmission in combination with the airframe of the helicopter. Such support structures are configured to transmit the dynamic and static longitudinal, lateral, vertical, and torsional loads developed by the main rotor assembly to the airframe of the helicopter at a single load transfer level (the transmission deck).
Representative embodiments of prior art main rotor assembly (MRA) support structures are illustrated in FIGS. 1A-1C. FIGS. 1A, 1B depict standpipe support structures SP having a plurality of attachment feet AF. FIG. 1A represents the configuration of a generalized embodiment of the standpipe support structure SP. The static rotor mast and the transmission of the main rotor assembly are integrated with the attachment collar AC of the standpipe support structure SP. FIG. 1B illustrates the particularized embodiment of the standpipe support structure SP currently utilized on Sikorsky Blackhawk and S-76 helicopters. In this type of standpipe support structure SP, the transmission housing TH of the main rotor assembly comprises the body of the support structure, including the attachment feet AF. The static rotor mast is integrated with the attachment collar THAC of the transmission housing TH.
Each of the above-described embodiments of MRA standpipe support structures SP is secured to the transmission deck of the helicopter by means of bolts passing through the attachment feet AF. The dynamic and static loads of the main rotor assembly are transmitted to a single load transfer level of the airframe (the transmission deck) via the attachment feet AF. Prior art standpipe support structures offer several advantages, including relative ease and low cost of fabrication as an integral unit, and ease of attachment to the transmission deck. Due to the relatively uncluttered configuration of prior art standpipe support structures SP, hydraulic lines, subsystem wiring, and other interfacing elements typically routed over the transmission deck may be readily run over/adjacent the exterior surface of prior art standpipe support structures SP.
On the other hand, prior art standpipe support structures SP are disadvantageous in several respects. Due to the manner of integration of the transmission with the standpipe support structure, the transmission housing acts as a structural member through which the dynamic and static loads of the main rotor assembly are intermediately transmitted. Moreover, the weight of the standpipe support structure is relatively large because of the high structural strength required of the support structure.
The high strength requirement is due primarily to the low profile configuration of the support structure and the effective load points of the main rotor assembly dynamic and static loads to which the standpipe support structure SP is subjected. The effective load points of the dynamic and static loads of the main rotor assembly have relatively large moment arms with respect to the low profile support structure, see FIG. 1D, which intensifies the loading effects experienced at the attachment feet AF (concomitantly, these loading effects also necessitate an increase in airframe structural strength in the load transfer zones (hardpoints) of the transmission deck). In addition, each of the attachment feet AF must be sized to accommodate the ultimate flight load conditions (fail-safe redundancy) and crashworthiness high mass retention (controlled displacement of the main rotor assembly in crashes).
Routine maintenance is more time consuming and labor intensive for prior art standpipe-type support structures SP. Any type of transmission maintenance (including removal of the transmission) requires removal of the rotor head, mast, and associated components prior to initiation of maintenance. The removed components must be reassembled and the operation thereof checked after completion of any routine transmission maintenance/overhaul.
An MRA strut support structure ST is illustrated in FIG. 1C. The strut-type support structure ST is currently utilized on the McDonnell-Douglas Apache attack helicopter. The strut support structure ST is a high profile configuration comprising an integration member IM, and a plurality of struts S such as cylindrical rods or machined legs extending from the integration member IM and terminating in attachment feet AF.
The static rotor mast of the main rotor assembly is attached to the integration member IM in a manner similar to the standpipe support structure ST. The transmission, however, is attached in suspended combination to the underside of the integration member IM, and consequently, is not part of the transmission path for the dynamic and static loads of the main rotor assembly. The attachment feet AF of the MRA strut support structure ST are utilized to secure the support structure to the airframe of the helicopter and to transfer dynamic and static loads of the main rotor assembly to respective hardpoints of a single level load transfer plane (transmission deck).
Due to the high profile configuration of the MRA strut support structure ST, however, the dynamic and static loads of the main rotor assembly are transmitted to single load transfer plane through the integration member IM. In consequence, the loading effects at the attachment feet AF (and in the respective hardpoints of the transmission deck) are not as severe as those experienced in the standpipe-type support structure ST.
Each of the struts S must be equally sized to react the maximum longitudinal, lateral, vertical, and torsional dynamic loads developed by the main rotor assembly, which increases the overall weight of the MRA strut support structure ST. Moreover, in light of the high profile configuration of the strut support structure ST, each strut S must be oversized to accommodate not only the dynamic loading developed by the main rotor assembly, but also the bending stresses that may be experienced in the event of the loss of any one strut S (fail-safe redundancy).
The routing of hydraulic lines, electrical subsystem wiring, and other interface components along the transmission deck is complicated by the network of struts S. Transmission maintenance and/or removal is simplified to the extent that access to the transmission does not require removal of the main rotor head, static rotor mast, and associated components. However, extraneous time and labor is required for transmission maintenance/removal inasmuch as one set of struts S must be removed to access the transmission. The configuration of MRA strut support structure ST is more complex than the MRA standpipe support structure SP, thereby requiring more time and labor to install and/or remove the strut-type support structure ST.
A need exists for a MRA support structure that has a relatively simple configuration with a relatively high profile. The support structure should be lightweight, inexpensive, relatively simple to fabricate, and readily secured to the helicopter airframe. Such a support structure should have a configuration that facilitates integration of the main rotor assembly therewith in such a manner that the engine transmission is non-structural (does not act as a transmission path for dynamic and static loads of the main rotor assembly). The support structure should provide fail-safe redundancy (ballistic tolerance) and crashworthiness high mass retention without a corresponding increase in structure weight.
The support structure should facilitate access to the transmission for maintenance/removal, i.e., not require removal of components of the main rotor assembly. The support structure should also facilitate routing of hydraulic lines, electrical subsystem wiring, and other interface components along the transmission deck.